1. Field of the Invention
The present invention relates to spherical ferrite nanoparticles and a method for producing the spherical ferrite nanoparticles. The spherical ferrite nanoparticles are configured such that the sizes of the spherical ferrite nanoparticles are set almost equal to one another. The producing method is carried out in an aqueous water solution.
2. Background of the Invention
Ferrite particles dispersed in an aqueous solution can be employed for various uses and in various fields. Among the various uses and fields, the ferrite particles often and intensely applied for biotechnology or medical care so that various advanced requirements are imposed on the ferrite particles.
As the ferrite particles for biotechnology and medical care, such ferrite particles as having respective spherical shapes and respective sizes within a range of several ten nm through several hundred nm are required. Moreover, the size distribution in the ferrite particles is required to be narrowed so that the sizes of the ferrite particles are set almost equal to one another.
Since the size distribution in the ferrite particles is set in the order of several ten nm, the ferrite particles can pass through various portions of a living body. If the ferrite particles have respective polyhedral crystalline shapes, the ferrite particles can not be always modified uniformly by biological molecules or polymeric molecules because the property of each of the ferrite particles becomes different dependent on the kind of crystal plane of the polyhedral crystalline shape thereof. However, if the ferrite particles have respective spherical crystalline shapes, the ferrite particles can be modified uniformly so that each of the ferrite particles can be surface-modified uniformly by biological molecules or polymeric molecules. Moreover, if the ferrite particles are only crystalline particles, the ferrite particles may be adhered with one another at the crystal planes thereof or crystal sides thereof because the ferrite particles have respective crystal planes, thereby causing the aggregation of the ferrite particles easily. As described above, however, if the ferrite particles have respective spherical crystalline shapes, the ferrite particles are not likely to be aggregated and thus separated easily, thereby enhancing the dispersion of the ferrite particles in an aqueous solution.
Since the biochemical substance is normally operated in an aqueous solution, the ferrite particles to be modified with biological molecules or polymeric molecules are particularly required to be suitable for the dispersion in the aqueous solution. In this point of view, the surface property of each of the ferrite particles is desired to be suitable for the dispersion in the aqueous solution while each of the ferrite particles has its spherical shape.
If the ferrite particles with a uniform size are sufficiently dispersed in an aqueous solution, the thus obtained dispersion liquid can be introduced into a living body such that the dispersed ferrite particles can be moved smoothly in the living body.
The ferrite particles, which are easily dispersed in an aqueous solution, with a uniform particle size of several ten nm through hundred nm and respective spherical shapes as mentioned above are desired as magnetic particles suitable for the application in various biochemical fields and medical care fields. However, a producing technique for producing the magnetic particles has not yet established.
As of now, various synthetic methods capable of producing a large amount of ferrite nanoparticles has been developed. For example, raw material powders are mixed, fired and mechanically crushed to form the ferrite nanoparticles. However, such a producing method has an advantage of excellent productivity but has a disadvantage of nonuniform particle shape and size distribution. Moreover, it is proposed that a metallic raw material is evaporated in inert gas atmosphere so as to form the metallic nanoparticles through the cooling and aggregation of the metallic raw material by the collision with the inert gas and thus form the intended ferrite nanoparticles synthesized through the oxidation under low pressure oxidation atmosphere. Such a producing method is called as “metallic evaporation oxidizing method”. According to the metallic evaporation oxidizing method, the intended ferrite nanoparticles can be shaped spherical but cannot be rendered uniform in particle size. The ferrite nanoparticles synthesized by the metallic evaporation oxidizing method are commercially available as “NanoTekR” made by C.I Kasei Co., Ltd. Furthermore, it is proposed that a raw material solution is splayed and the thus obtained mists are passed through high temperature area to be dried and synthesized, thereby forming the intended ferrite nanoparticles. Such a producing method is called as “splaying method” (Reference 1: JP-A 2002-025816). However, the splaying method has an advantage of capable of forming spherical ferrite nanoparticles but a disadvantage of large particle size distribution.
In this point of view, polyol method (Reference 2: B. Y. Lee, et al., Adv. Funct. Mater. 15 (2005)503.) and reverse micelle method (Reference 3: S. Sun and H. Zeng, J. Am. Chem. Soc. 124 (2002)8204.) are proposed. In these methods, the intended ferrite nanoparticles are synthesized in an organic solvent. According to these methods, the spherical ferrite nanoparticles with narrow particle size distribution can be synthesized, but the particles sizes of the spherical ferrite nanoparticles cannot be set to 16 nm or more and the spherical ferrite nanoparticles are covered with a surface-active agent. Since the surface-active agent is hydrophobicity, the spherical ferrite nanoparticles can be dispersed in an organic solvent, but cannot in an aqueous solution. In addition, if the ferrite nanoparticles are covered with the surface-active agent, it is difficult to immobilize biological molecules or polymeric molecules on the surfaces of the ferrite nanoparticles.
In view of the application of various biochemical fields and medical care fields, the ferrite particles easily dispersible in an aqueous solution are desired. Therefore, it is desired that the ferrite particles are synthesized in an aqueous solution.
As such a pioneer producing method as mentioned above, Reference 4 (Tamaura, et al.) and Reference 5 (Sugimoto et al.) can be exemplified. In Reference 4, the spherical ferrite particles can be obtained as part of water treatment process using the process that can precipitate ferrites from an aqueous solution. Concretely, FeSO4 and a small amount of sucrose are added into a deoxidized pure water and then NaOH is added into the deoxidized pure water. Then, the deoxidized pure water is heated and the thus obtained hydroxides are contacted with a CO2-removed air to form ferrite particles with a particle size within a range of 36 nm to 250 nm. In Reference 4, however, the size distribution of the ferrite particles becomes large. The inventors in this application surveyed Reference 4 so that the experimental results support the above-described results. Namely, the shapes of the ferrite particles can be made spherically but becomes random. Moreover, the particle size distribution in the ferrite particles becomes relatively large.
In Reference 5, the research about the particle configuration in the generation of iron oxides from hydroxide gels is described. Concretely, spherical ferrite particles within a particle size range of 30 nm to 1.1 μm can be obtained by generating the iron oxides from the hydroxide gel. In Reference 5, however, as shown in FIG. 7 of Reference 5, the particle sizes of the ferrite particles become almost uniform in the order of micro-meter, but not uniform in the order of less than 300 nm. Namely, the particle size distribution in the ferrite particles becomes relatively large in the order of less than 300 nm.
Accordingly, it is intensely desired to establish the producing method that can make the shapes of the ferrite particles spherical and can enhance the dispersion of the ferrite particles in an aqueous solution so that the ferrite particles can be suitable for biotechnology field and medical care field because as of now, such a producing method has not yet been established.    [Reference 1] JP-A 2002-025816    [Reference 2] B. Y. Lee, C. J. Bae, J. G. Park, H. J. Noh, J. H. Park and T. Hyeon, “Large-Scale Synthesis of Uniform and Crystalline Magnetite Nanoparticles Using Reverse Micelles as Nanoreactors under Reflux Conditions”, Adv. Funct. Mater. 15(3), pp. 503-509, (2005)    [Reference 3] S. Sun and H. Zeng, “Size-Controlled Synthesis of Magnetite Nanoparticles”, J. Am. Chem. Soc. 124, pp. 8204-8205 (2002)    [Reference 4] Y. Tamaura, G. S. Chyo and T. Katsura, “The Fe3O4-formation by the “ferrite process”: Oxydation of the reactive Fe(OH)2 suspension induced by sucrose”, Water research 13(1), pp. 21-31 (1979)    [Reference 5] T. Sugimoto and E. Matijevic, “Formation of Uniform Spherical Magnetite Particles by Crystallization from Ferrous Hydroxide Gels”, J. Colloidal and interface Science, 24(1), pp. 227-243(1980)    [Reference 6] JP-A 2006-219353    [Reference 7] Shimazu, Tada, Abe, Handa, “Control of Particle Size of Ferrite Nanoparticles by Seed Growth Method”, Extended Abstract of Japan Society of Powder Metallurgy Meeting, Heisei 18, Spring, 3-51A (2006)